Tin oxide coatings have been applied to glass to make it more antistatic. For example, such coatings have been applied to the surface of cathode ray tubes and similar devices, where the buildup of static charges is undesirable. Antistatic coatings combat the accumulation of dust on surfaces where dust impairs the function of the article.
Antistatic coatings are useful in other arts as well. For example, it is well known that numerous types of photographic film supports have the tendency to produce charges of static electricity during winding and unwinding, and that these charges do not easily dissipate, because the materials that are used as film supports usually are poor electrical conductors. High potentials that have been created in this manner, may discharge suddenly in the course of manufacture or in the course of utilization of the product by the user, causing flashes of light from static electricity and an undesirable recording of the static electricity discharge on a radiationsensitive layer, such as a layer of photographic emulsion. In order to avoid this result, it is customary in the prior art to apply to the backing of the film support an electrically conductive layer, also referred to as an antistatic layer, to facilitate the dissipation of the static charges, and thus avoid the sudden discharges and the resulting light flashes which otherwise would damage the radiation-sensitive layer. The antistatic coating need not be applied to the backing of the film support, as stated above. Alternatively, it may be applied in a subbing layer or in a conductive overcoat (COC).
Known antistatic layers generally consist of a binder in which is%dispersed an organic or inorganic conductive substance to render the surface on which the layer is coated, for example, a film support, sufficiently conductive to make possible the flow of the electrostatic charges to a discharge means. Most often, antistatic layers are more or less hygroscopic layers, the efficiencies of which vary as a function of the degree of humidity in the air. Some layers may not be very suitable for use under conditions of low relative humidity because they are no longer sufficiently conductive. Likewise, some hygroscopic layers are not very suitable at conditions of high humidity because they become sticky. It is difficult to separate them from the surfaces to which they adhere. The effort that is necessary to separate two superimposed, layers, for example, sometimes creates charges higher than those that would appear in the absence of any antistatic layer.
An object of this invention is to provide a tin oxide composition that has enhanced electroconductive properties. Another object is to provide antistatic coatings produced from the compositions of this invention. Still another object is go provide a low temperature method for producing antistatic coatings.
These objects have been satisfied by providing a composition which comprises tin oxide and boron oxide covalently bound in a polymeric network. These compositions are made in organic solvent solution by a process which does not produce a colloid, but which provides a solution of the inorganic polymer or oligomer. The solution can be used to prepare a coating on a substrate, i.e. the organic solvent can be used as a vehicle for spreading the polymer on the surface to be coated. The solvent can be readily removed by vaporization at low temperature. After solvent removal, the coating, film or layer is transparent, and provides antistatic properties to the surface or substrate being treated.
The coating process of this invention is inexpensive, and readily carried out without the need for costly machinery. Furthermore, it has been found that it is not necessary to cure the coating at elevated temperatures greater than about 150.degree. C. to reduce electrical resistivity. This finding, and the degree of antistatic property conferred by the coating, were entirely unexpected.